1. Field of the Invention
This invention relates to a hollow fiber membrane type artificial lung and a method for the manufacture thereof. More particularly, this invention relates to a hollow fiber membrane type artificial lung enjoying high permeability to gas, low cost and long service life, and to a method for the manufacture thereof.
2. Description of Prior Arts
Generally during the course of surgical operation of the heart, a hollow fiber membrane type artificial lung is used incorporated in an external circulatory path for the purpose of leading blood having a depleted oxygen supply out of the patient's circulatory system and oxygenating the blood. For use in artificial lungs of this class, two types of hollow fibers are available; homogeneous membrane type and porous membrane type. With the homogeneous membrane, passage of a gas through the membrane is effected by the molecules of the gas being dissolved and dispersed in the membrane. Polydimethyl siloxane rubber is a typical material for the homogeneous membrane. A Colobeau membrane type lung making use of this particular material has already been commercialized. In terms of the permeability to gas, however, silicone is the only material that has been demonstrated to be usable for the homogeneous membrane. Because of its strength, however, the silicone membrane cannot be obtained in any smaller thickness than 100 .mu.m. Because of this reason, the permeability of the silicone membrane to gas has its limit. This membrane shows poor permeability particularly to carbon dioxide gas. Silicone as a material has another disadvantage in that it is expensive and is deficient in processibility.
In contrast, the porous membrane permits a gas to flow in bulk through the micropores distributed therein because these micropores are very large as compared with the molecules of the gas being passed. Various types of artificial lungs using porous membranes such as microporous polypropylene membrane, for example, have been proposed. Since the porous membrane has high permeability to steam, there is a possibility that the moisture permeating the micropores will condense to form water and impair the performance of the membrane. When the porous membrane is used for circulation of blood over a protracted period, it at times suffers from the disadvantage that leakage of blood plasma occurs.
To eliminate the various drawbacks suffered by the conventional porous membrane as described above, there has been proposed a hollow fiber which has a thin gas-impervious film of methylhydrogen polysiloxane formed on the wall of a hollow fiber substrate containing in the wall thereof through micropores not exceeding 10 microns in diameter (Japanese Patent Publication No. SHO 54(1979)-17,052). In this hollow fiber, the coating of methylhydrogen polysiloxane is formed not merely inside the micropores distributed in the hollow fiber substrate but also on the internal and external surfaces of the wall of the hollow fiber substrate. Since the inside diameter of the hollow fiber substrate is decreased by the thickness of coating thus formed on the internal surface, the exchange capacity of the hollow fiber is proportionately lowered. By the same token, the amount (the thickness of coating) of methylhydrogen polysiloxane to fill the micropores is proportionately increased. As the result, the overall permeability of the hollow fiber to gases such as oxygen and carbon dioxide gas is lowered. This hollow fiber can be used advantageously in aqualungs. When it is used in an artificial lung for a protracted period, however, it will eventually suffer from leakage of blood plasma.